Ocean Heat Content
History of OHC Research
at the Rosenstiel School
Based on deliberations of the Prospectus Development Team 5 tasked by NOAA and the National Science Foundation (NSF), improving our understanding of hurricane intensity requires knowledge of the 1) atmospheric circulation; 2) inner core and eyewall processes; and, 3) upper-ocean circulation and ocean heat transport (Marks, Shay and the PDT-5 1998). While the oceanic energy source for tropical cyclones (TC) has largely been known for more than half of a century (Palmen 1948; Fisher 1958; Perlroth 1967; Leipper 1967), subsequent studies indicate that the maximum TC intensity is constrained by thermodynamic effects where the sea surface temperature (SST) is considered a major contributor (Miller 1958; Emanuel 1986).
Notwithstanding, research on the SST response to TC passage has been largely focused on the “negative” feedback aspects of how a cooled upper ocean affects the atmosphere by decreasing air-sea fluxes as the oceanic and lower atmospheric boundary layer temperatures approach an equilibrium value (Chang and Anthes 1978). That is, as the TC strengthens, winds induce more stress on the upper-ocean surface causing strong turbulent mixing across the base of oceanic mixed layer and to a lesser extent upwelling of the thermocline due to net wind-driven current transport away from the storm center (Price 1981; Gill 1982; Sanford et al. 1987; Shay et al. 1992). Shear-induced mixing effects deepen and cool the oceanic mixed layer (OML) as colder thermocline water is entrained from below. This entrainment heat flux subsequently causes the OML temperature (and by proxy the SST) to decrease, limiting air-sea heat and moisture fluxes that reduce TC intensity. This negative feedback mechanism is particularly effective when the OML depths are shallow or when storms become stationary for a few days. By contrast, in regimes where the OML (and the depth of 26°C water is deep), the OHC can be quite large (Leipper 1967; Leipper and Volgenau 1972; Shay et al. 2000). As this deeper OML has greater vertical extent, more turbulence is required to overturn and cool the deeper layers. A well-studied example of this effect is the response of Hurricane Opal (1995) that intensified rapidly as it crossed a warm core ring in the Gulf of Mexico (Shay et. al. 2000) under favorable atmospheric conditions (Bosart et al. 2000). When Opal encountered this deeper, warmer oceanic regime, the storm unexpectedly intensified from a Category 1 to a Category 4 hurricane status in 14 hours, as atmospheric conditions were favorable. Sensitivity studies with a coupled ocean-atmosphere model (Hong et al. 2000) showed that the central pressure of Opal was more than 10 hPa higher when the WCR was removed in the pre-storm state. Using a hurricane season (June through November) climatology, Mainelli (2000) extended the Opal investigations across the Atlantic Ocean basin including the Caribbean Sea and the Gulf of Mexico.
More recent examples included both hurricanes Katrina and Rita encountering the Loop Current and warm core ring in the central Gulf of Mexico in 2005 (Scharroo et al. 2005; Shay 2008). For both hurricanes, Shay (2008) showed that the sea-level pressure decreases were directly correlated to large values of the 26°C isotherm depth and OHC than just SSTs, which were essentially flat. Using the Statistical Hurricane Intensity Prediction Scheme (SHIPS: DeMaria et al. 2005), Mainelli et al. (2008) found that the OHC parameter contributed about 5 to 6% to the reduction in intensity forecasting using the seasonal climatological approach. Lin et al. (2009) showed that the translation speeds of typhoons may impact its intensity in that for fast movers (>6 m s-1) only 60 kJ cm-2 are necessary for intensification to severe status in the Western Pacific Ocean basin compared to more than 100 kJ cm-2 for slower moving typhoons. These findings support the premise that oceanic regimes with high OHC contribute to TC intensification where SST cooling is reduced (e.g., less negative feedback) beneath the storm and maintaining or enhancing surface sensible and latent heat fluxes.
Consistent within this framework, recent improvements to SHIPS have shown that OHC parameter has reduced intensity forecast errors in the Atlantic Ocean Basin (DeMaria et al. 2005, Mainelli et al. 2008). These studies have shown that a seasonal OHC climatology reduced forecast errors in intensity by an average of 2% when averaged over all storms between 1995 and 2003 compared to less than 1% from an annual analysis (Goni and Trinanes 2003). Notwithstanding, if only the western part of the Atlantic Ocean basin is used (60 to 100°W), intensity errors are reduced from 3 to 6% when averaged over all storms. In some cases, the reduction in intensity errors is considerably more dramatic as observed during hurricane Ivan in 2004. SHIPS with seasonal OHC showed as much as a 22% reduction in forecast intensity errors (Mainelli et al. 2008). This result points to the importance of OHC variability in the warm pool of the Caribbean Sea and the Gulf of Mexico’s Loop Current (LC) and warm core rings (WCR) as well as other eddy-rich regimes such as the western Pacific Ocean (Lin et al. 2005; Ali et al. 2007).
The Eastern Pacific Ocean basin (hereafter referred to as EPAC) is also a region of significant upper oceanic variability given the warm pool and gradual shoaling of the oceanic thermocline from west to east, and the westward propagation of WCRs forced either by low-level jets (Hurd 1929; Kessler 2003) or current instabilities (Hanson and Maul 1991) (Figure 1). This feature moved southwestward at 13 to 15 cm s-1 and dissipated within the Eastern Pacific Investigation of Climate (EPIC) domain in late Oct 2001 (Figure 2). Over this oceanic regime, tropical cyclogenesis often begins between 10 and 14°N and 90 to 100°W, which is characterized by SST gradients and OHC variations that impact TC intensity change (Raymond et al. 2004). Of particular importance to the maintenance of the SST and OML temperature structure are the sharp thermal gradients across the OML base starting about 30 to 35 m beneath the surface and the 20°C isotherm separates the upper from the lower layer in a two-layer model. During hurricane Juliette in Sept 01 (Shay and Jacob 2006), and the subsequent intensification to category 4 status, the SST cooling was less than 1°C in the regime with strong vertical gradients (~20 to 24 cph: cycles per hour) (Wijesekera et al. 2005). Wind-driven ocean current shear tends to be insufficient to significantly cool the upper ocean through shear instability until Juliette moved into an area with weaker stratification (~10 cph) where SST cooling was 4 to 5°C. Entrainment mixing across the OML base due to ocean current shear did not lower the bulk Richardson number to below criticality. Hence, a larger fraction of OHC was available for Juliette through air-sea fluxes during the hurricane’s rapid intensification phase.
This investigation updates the original approach by Shay et al. (2000) and Mainelli (2000) in developing a seasonal TC climatology to monitor thermocline depths and OHC from multiple radar altimetry platforms. This approach differs from that of Goni and Trinanes (2003) in that they form an annual climatology to represent a global TC heat potential throughout the entire year. Since TC’s only occur in specific seasons in both the northern (Boreal summer) and southern hemisphere (Boreal winter), upper ocean thermal energy is smeared over an entire year compared to a TC season that may lead to significant over (under) estimations in the southern (northern) parts of the domains compared to thermal energy available to TC’s in each basin as observed in the western Pacific Ocean.
The 20°C isotherm depth is used as the level for a two-layer model to estimate reduced gravities (i.e., density differences between the upper and lower ocean layers) (O’Brien and Reid 1967; Kundu 1991; Goni et al. 1996). Radar altimetry data are merged and blended each day when new SHA data become available. Two or three sets of altimeter data are then objectively analyzed to the same grid as the hurricane season climatology derived from US Navy’s Generalized Digital Environmental Model (GDEM) for the two-layer model application (Teague et al. 1990). The resultant isotherm depths (particularly the 26°C isotherm) are used to estimate OHC when combined with sea surface temperature (SST) and a climatological OML depth over the hurricane season. The isotherm depths and OHC estimates are carefully compared to temperature structure measurements from the Tropical Atmosphere Ocean (TAO) mooring data spanning the Eastern Pacific Ocean equatorial wave guide, EPIC, and Volunteer Ship of Opportunity XBT transects in building an evaluated hurricane season climatology in the EPAC (May through November). Finally, a stratification parameter is introduced to provide a measure of basin-to-basin OHC variability and the strength of the vertical density gradients to form an equivalent OHC